1. Field of the Invention
This invention relates to a novel dipyrromethene-metal chelate compound, and an optical recording medium using thereof which can perform recording and regenerating with a higher density than that of the prior art.
2. Description of the Related Art
To date, a DVD with a capacity of 4.7 GB has been developed and marketed as an optical recording medium with a larger capacity than a CD. Since the DVD is a read-only medium, there has been needed an optical recording medium capable of recording and regenerating comparable to the capacity. Among others, a rewritable type is called a DVD-R.
In a DVD for high-density recording, an oscillation wavelength of a laser is 630 nm to 680 nm which is shorter than that for a CD. As dyes for an organic-dye optical recording medium for such a shorter wavelength, cyanine, azo, bezopyran, benzodifuranone, indigo, dioxadine, porphyrin dyes, etc. have been suggested in, for example, JP-A 4-74690, JP-A 5-38878, JP-A 6-40161, JP-A 6-40162, JP-A 6-199045, JP-A 6-336086, JP-A 7-76169, JP-A 7-125441, JP-A 7-262604, JP-A 9-156218, JP-A 9-193544, JP-A 9-193545, JP-A 9-193547, JP-A 9-194748, JP-A 9-202052, JP-A 9-267562 and JP-A 9-274732. There have been, however, solved various problems such as poor durability, those inherent to use of a short wavelength including a poor jitter due to a larger distributed pit formation caused by much influence on the surrounding area whereas a small pit must be formed with a focus laser beam, deteriorated crosstalk in a radius direction, a poor modulation degree due to an extremely small pit or reduction in a reflectance or sensitivity caused by selecting an organic dye having an inappropriate optical constant such as a refractive index and an extinction coefficient for a desired laser wavelength in a recording layer.
Furthermore, as in increase of a recording speed in a CD-R, there has been desired to provide an optical recording medium which can deal with recording at double speed or more compared with usual recording speed for a DVD-R. However, there remain the problems such as poor recording sensitivity associated with high-speed recording and a poor jitter.
We have already suggested an optical recording medium using a dipyrromethene-metal chelate compound in, for example, JP-A10-226172, JP-A11-092682, JP-A11-165465, JP-A11-227332, JP-A11-227333 and JP-A11-321098. However,there has not been solved a problem of deterioration in recording properties associated with the above high-speed recording, and thus, further improvement has been needed.
Thus, an objective of this invention is to provide a highly durable optical recording medium capable of recording and regenerating with a short wavelength of laser with a wavelength of 520 to 690 nm and suitable to high-density and high-speed recording.
We have intensely investigated an optical recording medium using a dipyrromethene-metal chelate compound disclosed in, for example, JP-A 10-226172 above and have finally found that for a dipyrromethene-metal chelate compounds, a particular substituent may be selected to provide an optical recording medium which is excellent in recording properties and durability as well as can deal with recording at a higher speed than a recording speed in the prior art, to achieve this invention. Thus, this invention relates to:
[1] An optical recording medium comprising at least a recording layer and a reflecting layer on a substrate wherein the recording layer contains at least one dipyrromethene-metal chelate compound represented by general formula (1): 
wherein R1 to R6 independently represent hydrogen, halogen, nitro, cyano, hydroxyl, amino, carboxyl, sulfo, substituted or unsubstituted alkyl with up to 20 carbon atoms, alkoxy, alkylthio, aryloxy, arylthio, alkenyl, acyl, alkoxycarbonyl, carbamoyl, acylamino, aralkyl, aryl or heteroaryl; R7 represents halogen, aryl, heteroaryl, alkoxy, alkylthio, aryloxy or arylthio; A represents substituted or unsubstituted aromatic or heterocyclic ring with up to 20 carbon atoms; L1 represents substituted or unsubstituted bivalent residue forming a ring together with carbon atoms to which it attaches and optionally containing a hetero atom; and M1 represents transition metal element;
[2] The optical recording medium as described in [1] wherein the dipyrromethene-metal chelate compound is a dipyrromethene-metal chelate compound represented by general formula (2): 
wherein R8 to R3 independently represent hydrogen, halogen, nitro, cyano, hydroxyl, amino, carboxyl, sulfo, substituted or unsubstituted alkyl with up to 20 carbon atoms, alkoxy, alkylthio, aryloxy, arylthio, alkenyl, acyl, alkoxycarbonyl, carbamoyl, acylamino, aralkyl, aryl or heteroaryl; R14 represents halogen, aryl, heteroaryl, alkoxy, alkylthio, aryloxy or arylthio; B represents substituted or unsubstituted aromatic or heterocyclic ring with up to 20 carbon atoms; L2 represents substituted or unsubstituted alkylene residue forming a ring together with carbon atoms to which it attaches; and M2 represents transition metal element;
[3] The optical recording medium as described in [2] wherein the dipyrromethene-metal chelate compound is a dipyrromethene-metal chelate compound represented by general formula (3): 
wherein R15 to R20, R22 to R25 independently represent hydrogen, halogen, nitro, cyano, hydroxyl, amino, carboxyl, sulfo, substituted or unsubstituted alkyl with up to 20 carbon atoms, alkoxy, alkylthio, aryloxy, arylthio, alkenyl, acyl, alkoxycarbonyl, carbamoyl, acylamino, aralkyl, aryl or heteroaryl; R21 represents halogen, substituted or unsubstituted aryl with up to 20 carbon atoms, heteroaryl, alkoxy, alkylthio, aryloxy or arylthio; M3 represents transition metal element;
[4] The optical recording medium as described in [1] wherein R1 in general formula (1) is halogen;
[5] The optical recording medium as described in [2] wherein R8 in general formula (2) is halogen;
[6] The optical recording medium as described in [3] wherein R15 in general formula (3) is halogen;
[7] The optical recording medium as described in any of [1] to [6] wherein the recording layer further contains at least one dipyrromethene-metal chelate compound represented by general formula (4): 
wherein R26 to R33 independently represent hydrogen, halogen, nitro, cyano, hydroxyl, amino, carboxyl, sulfo, substituted or unsubstituted alkyl with up to 20 carbon atoms, alkoxy, alkylthio, aryloxy, arylthio, alkenyl, acyl, alkoxycarbonyl, carbamoyl, acylamino, aralkyl, aryl or heteroaryl; R34 represents halogen, substituted or unsubstituted aryl with up to 20 carbon atoms, heteroaryl, alkoxy, alkylthio, aryloxy or arylthio; and M4 represents transition metal element;
[8] The optical recording medium as described in any of [1] to [7] wherein the recording layer has a refractive index of at least 1.8 at a laser wavelength and an extinction coefficient of 0.04 to 0.40;
[9] The optical recording medium as described in any of [1] to [7] wherein recording and regenerating can be performed for a laser beam with a wavelength within a range of 520 to 690.
This invention also relates to a dipyrromethene-metal chelate compound defined in any of the above [1] to [6].
A dipyrromethene-metal chelate compound can be used as a recording layer to provide a highly-durable rewritable optical recording medium capable of recording and regenerating using a laser with a wavelength of 520 to 690 nm and suitable to high-density and high-speed recording which is considerably expected to be as a high-density recording medium.